R-T-E cereals, especially puffed cereals are popular food items. Generally, puffable cereal pellets or grains are puffed in cereal puffing guns by heating under pressure with steam to high temperatures and then being discharged from the gun to a zone of lower pressure. The change in pressure causes an explosive vaporization of the superheated moisture in the pellet which in turn causes the pellet to puff. A variety of such gun puffing apparatus are well known and used by R-T-E cereal processors. For example, a gas heated "C-gun" or continuous puffing gun is described in U.S. Pat. No. 3,656,965, issued to Strommer et al. entitled "Process and Apparatus for Controlling the Expansion of Puffable Materials," which is incorporated herein by reference. Electrically heated guns are also used. (See for example, U.S. Pat. No. 3,972,274, issued to Tsuchiya entitled "Apparatus for Continuously Treating Particulate Material" and U.S. Pat. No. 4,265,922, issued to Tsuchiya et al., May 5, 1981 entitled "Induction Heating Method for Processing Food Material").
The continuous puffing guns in commercial operations generally employ a fixed orifice exit nozzle. Fixed orifice nozzles are used due to the ruggedness and simplicity with which a fixed orifice area nozzle can be constructed.
The exit nozzle orifice cross-sectional area, hereafter called the nozzle orifice area, is an important process variable in the operation of continuous puffing guns because this area ultimately determines the range of pellet feed rates that can be processed as well as other important process operating aspects.
To operate the gun and produce a satisfactory puffed product, the steam flowing through the gun must supply the appropriate amount of heat to the pellets. If too little heat is supplied, the final product moisture will be high, causing the bulk product density to be high, and/or the steam may condense, causing the gun to stop operating. If too much heat is supplied, the product moisture will be low, causing the product to fracture and creating large amounts of fine product waste. The amount of heat supplied to the pellets by the steam depends on the steam enthalpy and the steam flowrate.
The steam enthalpy is a function of the steam temperature and pressure. However, the steam temperature is the primary controlling variable for the steam enthalpy because the effect of the steam temperature on the steam enthalpy is much greater than that of the steam pressure. Consequently, at a given steam flowrate, the heat supplied to the pellets can be controlled by the steam temperature within the operating limits of the steam superheater. Control over the amount of heat necessary to process the pellets throughout the total operating range of continuous guns is not obtainable by controllably varying the steam enthalpy alone. Therefore, the steam flowrate must also be varied over the total operating range of continuous guns.
The steam flowrate is a function of the steam temperature, steam pressure and nozzle orifice area. However, because the steam temperature is the primary controlling variable for the steam enthalpy, and because the steam temperature has only a minor effect on the steam flowrate, the steam temperature is not a practical controlling variable for the steam flowrate. Likewise, the steam pressure cannot be used to independently control the steam flowrate, because it is the primary controlling variable for the puffed product size. Consequently, the only variable that can independently control the steam flowrate is the nozzle orifice area.
As already mentioned, however, the nozzle orifice area is essentially fixed because production must be interrupted to change the nozzle. Hence, the continuous puffing guns have limited operational flexibility. In addition, an optimal nozzle size exists for any given pellet feedrate. Since the nozzle size is fixed, a compromise size, which is usually suboptimal, is chosen. Moreover, employing a fixed orifice area nozzle does not allow the gun to be adjusted for minimal energy usage. Thus, the disadvantages of fixed orifice nozzles include product L variability, suboptimal quality, operational inflexibility and excess energy usage.
Accordingly, a nozzle that continuously adjusts the orifice area thereby is capable in turn of controlling the steam flowrate which control could improve product quality, increase operational flexibility and minimize energy usage.
A variety of exit nozzles of adjustable orifice area have been developed for food processing apparatus that involve heat treatment with high pressure steam. For example, several exit nozzles are known which had been developed for protein texturization apparatus (see, for example, U.S. Pat. No. 3,707,380, issued Dec. 26, 1972 and in particular U.S. Pat. No. 3,776,470, issued Dec. 4, 1973 each to T. Tsuchiya entitled "Variable Nozzle"). While useful in connection with protein texturization processes, these nozzles are unsuitable for use as exit nozzles for continuous cereal puffing apparatus for two reasons. First, in terms of functionality, the known nozzles are primarily adapted to open and close momentarily (although they do not close completely) rather than continuously controlling the orifice cross-sectional area over a small range of areas. Second, these nozzles are insufficiently rugged to withstand the rigors of use in connection with a commercial scale continuous gun puffing apparatus.
It is to be appreciated that commercially useful exit nozzles must be extremely rugged and durable. The nozzles must be designed to withstand the shock waves created as the steam exits the apparatus at supersonic velocities. The shock waves are caused by the exiting steam traveling at supersonic velocity. Useful devices must be designed so as not to fail from metal fatigue or from physical wear.
The prior art additionally includes a number of variable nozzles designed to operate in a hostile environment, especially those developed for rocket engines. In particular, a nozzle described in U.S. Pat. No. 3,398,536 (issued Aug. 27, 1968 to A. B. Stolins, Jr., entitled "Fluid Flow Nozzle Having Temperature Compensating Means") employs two nozzle pieces with eccentric cylinders to create a nozzle with the proper orifice area. However, nozzles of this design are not suitable for use as an exit nozzle for cereal puffing apparatus. The nozzle design of the '536 patent includes a mating surface between the two valve control pieces which is normal, i.e., perpendicular to fluid flow, which configuration can damage the cereal product as it passes through the nozzle even in a fully open position. Of course, whether the structure comprises a valve (i.e., reduction of the orifice area to zero) or a nozzle (i.e., a reduction of the orifice area to a size greater than zero) depends upon minor differences in the shape of the eccentric bore size and shape. Moreover, the nozzle while variable is not continuously variable but remains fixed once the orifice opening is fixed.
Given the state of the art, there is a continuing need for new and useful exit nozzles for cereal puffing apparatus. Accordingly, it is an object of the present invention to provide an exit nozzle capable of continuously varying the orifice cross-sectional area.
It is another object of the present invention to provide an exit nozzle of simplified design having few moving parts.
Still another object of the present invention is to develop an exit nozzle of enhanced durability.
Still another object of the present invention is to provide an exit nozzle readily adaptable to automatic control.
Still another object of the present invention is to provide an exit nozzle having self-cleaning, smooth internal surfaces.
Still another object of the present invention is to provide an exit nozzle that does not have an internal control surface at right angles to fluid flow so as to provide a nozzle having a reduced potential to damage fluids flowing therethrough.
Surprisingly, the above objectives can be realized and superior nozzles and valves can be fabricated by structure designs having valve mating confronting surfaces at other than a plane at 90.degree. to fluid flow, e.g., a 45.degree. cone or parabola.
Additional objects and advantages of the present invention are described in detail below.